Fixes, improvements and @BuiltValueHook

New features:

• Add @BuiltValueHook annotation. It provides the same functionality as
  _initializeBuilder and _finalizeBuilder, but in a more visible way:
  annotate a static method on the value class with @BuiltValueHook to
  have it called on builder creation or finalization.
• Add back serializeNulls to BuiltValueSerializer annotation. By default
  generated serializers skip null fields instead of writing them; set
  serializeNulls to write the nulls.

New minor functionality:

• Support use of nulls in collections when the key or value types are
  explicitly nullable.
• Allow JsonObject to be instantiated from a Map<dynamic, dynamic>.
• Mark nested builder getters in instantiable: false classes not nullable,
  to match the implementations. Use autoCreateNestedBuilders: false to get
  the old behaviour.
• Allow explicit nulls in JSON for nullable fields when deserializing.
• Specify annotation targets; the analyzer will now hint if a built_value
  annotation is used where it has no effect.
• Support polymorphism with mixed in parent value type: generated builder
  now mixes in parent builder.

Bug fixes:

• Fix support for serializing and deserializing nulls.
• Fix nestedBuilders: false with instantiable: false.
• Fix enum deserialization fallback for int.
• Annotating a wrong type getter with @SerializersFor is now an error,
  instead of just generating nothing.

Cleanup:

• Removed Angular mixin from example, as this feature is no longer needed:
  Angular now directly supports using static members in templates.